Tuesday, February 01, 2011

A Hole In The Wall Is Not A Barrier

Some time I am so picky, I hate myself! :)

This has all the good intentions, but the visual picture it has given is not correct. A student is attempting to demonstrate a "potential barrier" that we often have to deal with in a quantum mechanics class. She does this by creating a "snow sculpture" that consists of a wall of snow. But to show that an electron an tunnel through the barrier, she created a hole in the wall!

So the question is, is this a valid and accurate-enough visual representation of the phenomenon?

I claim that it is not. When I used to do point-contact tunneling, we pushed the tip onto the insulating barrier until we see a tunneling current. However, depending on how fragile the insulating barrier is, or how much force you push with the tip, sometime the barrier cracks and forms pinholes. These pinholes will then create Ohmic contacts and basically a short, where the current flow is the normal conduction. You no longer see tunneling current.

So when I look at the video in this demonstration, I can't help but see this hole in the barrier as not an illustration of electron tunneling, but rather, an Ohmic contact.

3 comments:

How would you use snow to explain tunneling to a layman, then? I actually thought it was a decent analogy, and a funny prank!

Perhaps a more accurate way would have been to lightly pack the tunnel with snow so that it 'gave way' as you (the electron) pushed through it, and then a mechanism to 'repack' the snow behind you. Which would have been too much work for a fun snow day. :)

Perhaps I'm not looking at it the way you are, but an ohmic contact seems to be a lot more complicated than a simple hole through a snow wall. How can you guarantee correct I-V characteristics? Which parts of the snow system corresponds to different parts of the ohmic contact system?

Maybe there is no way of explaining tunneling to a layman using snow! Just because one has some raw material, doesn't mean that one can turn it into a sports car.

The problem in such demonstration is that it gives the impression that the electrons are moving through openings and crevices in the barrier. This is certainly NOT what is going on in tunneling. This is where the term "tunnel" as used in the pedestrian sense is not the same as the one used in quantum mechanics.

The "hole" is the ohmic contact. Without the snow barrier, people (electrons) move freely where the medium of movement is air (conductor). With the snow barrier, the hole now becomes a path where people (electron) can still move in a medium of air (conductor). This is an Ohmic movement. They may not move as fast as if the barrier is not there, but this is similar to charges moving through a conductive constriction. The constriction will have a higher resistance, but it is still ohmic and not tunneling.

I don't know how one can easily demonstrate tunneling to the general public. But I certainly do not want to construct something that give a visually incorrect idea of what it is. Often, such things can lead to more harm than good.